I see cheating itself an act that should allow you to justifibly kill the person. Saudi Arabia has the right idea when it's legal in their country to kill a person for cheating on you. The amount of damage done in that one act can take someones life without killing them.

As for who's fault is it that is entirely circumstancial based on the situation at hand there is no clear cookie cut of one or the other. Every relationship has so many variables it had to be viewed case by case.